1. Field of the Invention
The present invention relates to night vision devices. More specifically, the present invention provides a combined day and night sight capable of providing an intensified and potentially magnified sight picture at night or under poor viewing conditions, and a potentially magnified sight picture in more favorable light and viewing conditions.
2. Description of the Related Art
The use of a light source to illuminate the target—whether visible light or infrared—has the disadvantage of indicating the location of the shooter. Laser aiming devices carry the same disadvantage. Thermal infrared imaging, which does not require a light source, has the disadvantage of producing an unnatural image. Light gathering and intensifying night vision devices produce natural images, but are only useful to the extent that there is available light to gather and intensify. The use of tritium illuminated sights, while making the sights more visible, fails to illuminate the target even if the tritium-illuminated aiming reticle is within a telescopic sight providing light gathering and magnification. Additionally, both night vision equipment and daytime telescopic sight add weight and bulk to the weapons with which they are used.
A typical night vision sight utilizes an objective lens having a maximized size for maximum light gathering capability. After passing through the objective lens, light passes through a focusing assembly that is used to vary the distance light travels between lenses within the sight by moving either a focal length adjust lens with respect to the objective lens, or a mirror within the night vision device along an axis to change the length of the light path. The light will therefore be brought into sharp focus on the photosensitive surface of the image intensifier.
A photocathode having an electrical current flowing therethrough which forms the photosensitive surface of the image intensifier converts the optical image into an electronic image that is transmitted through an electron flow. The electrons are accelerated through the image intensifier, and remain focused because of the close proximity of surfaces within the image intensifier tube. Acceleration of the electrons, combined with a microchannel electron-multiplying plate, result in intensification of the original image. When the electrons reach a screen, the electronic image is converted to an optical image. The final, amplified visible image is displayed to the shooter or to other optical devices within the night sight.
The output image of the image intensifier may be presented to a collimator. The collimator presents the image to the shooter in a manner so that it appears to originate from a long distance. The orientation of the collimator affects the degree to which the image transmitted by the collimator appears to originate from the same angle at which it entered the objective lens.
Some night sights attempt to combine a daytime telescopic scope and a night sight into the same sight. Many of these devices use two optical paths, one used during the day, and one for use at night. In addition to making the sight unnecessarily heavy, the use of two optical paths creates a likelihood of differences between point of aim and point of impact from one optical path to the other.
Other night vision devices utilize a removable image intensifier tube to permit use of the night sight as a day sight. However, image intensifier tubes are delicate instruments, subject to damage when removed. Furthermore, a temperature difference between the image intensifier and the scope upon insertion of the image intensifier may cause fogging of the sight.
Other night vision sights utilize an image intensifier moved within the housing from a position within the sight's light path to a position outside the sight's light path depending on whether the sight is being used during the day or at night. Such systems may also require the movement of the objective lens and the eye piece lens assembly in conjunction with the image intensifier tube so that the daytime optics move outward as the image intensifier tube is moved therebetween, thereby maintaining the proper focal distance for the daytime optics. Such movement of both the daytime optics and the image intensifier tube raises the issue of the repeatability of these movements.
Accordingly, there is a need for a combined day and night sight forming a single compact, lightweight package that minimizes the bulk and weight that the user must deal with. There is a further need for a combination day/night sight having no moving optical elements, no moving mirrors, and no compromise of the environmental seal. There is an additional need for a night sight wherein any beam splitters are not critical to the operation of the sight itself, but are used for unrelated functions such as optical designating, detection, communications, or ranging functions.